U.S. patent number 5,911,898 [Application Number 08/450,712] was granted by the patent office on 1999-06-15 for method and apparatus for providing multiple autoregulated temperatures.
This patent grant is currently assigned to Electric Power Research Institute. Invention is credited to Cristian Filimon, Stephen M. Jacobs.
United States Patent |
5,911,898 |
Jacobs , et al. |
June 15, 1999 |
Method and apparatus for providing multiple autoregulated
temperatures
Abstract
The present invention generally relates to use of constant
current power supply to control temperatures of a device to plural
Curie temperatures, without sacrificing the precision and
uniformity of temperature achieved in the known devices. In
accordance with exemplary embodiments, multiple layers of alloy
having different Curie temperatures are separately accessed as an
outer most layer is heated through its Curie point. Power to the
device can be controlled by varying a frequency of circulating
current and by searching to identify a layer of Curie point
material which provides heating at a temperature accurately
controlled to a fixed value, where any one of a number of different
such temperatures may be selected.
Inventors: |
Jacobs; Stephen M. (Cupertino,
CA), Filimon; Cristian (Los Gatos, CA) |
Assignee: |
Electric Power Research
Institute (Palo Alto, CA)
|
Family
ID: |
23789199 |
Appl.
No.: |
08/450,712 |
Filed: |
May 25, 1995 |
Current U.S.
Class: |
219/505; 219/486;
219/634; 219/504; 219/553 |
Current CPC
Class: |
H05B
6/106 (20130101); H05B 2206/023 (20130101) |
Current International
Class: |
H05B
6/10 (20060101); H05B 6/02 (20060101); H05B
001/02 () |
Field of
Search: |
;219/504,505,501,494,497,553,660,665,667,661,634,483-486
;338/22R,22SC |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Paschall; Mark
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis,
L.L.P.
Claims
What is claimed is:
1. Apparatus for generating heat comprising:
means for generating a relatively constant current; and
means for selectively producing heat at any one of plural
relatively constant temperatures in response to said constant
current generating means, based on reflected resistance of said
heat producing means, said heat producing means including:
a core having at least one electrically conductive, non-magnetic
material; and
at least two layers of magnetic material, a first of said at least
two layers having a first Curie temperature and a second of said at
least two layers having a second Curie temperature different from
said first Curie temperature, said core being cooperatively
arranged with said first layer and said second layer to selectively
produce heat at said first Curie temperature and said second Curie
temperature.
2. Apparatus according to claim 1, wherein said constant current
generating means further includes:
means for detecting an operating point of said heat producing
means;
means for controlling said constant current generating means to
maintain operation at said operating point; and
means for selectively adjusting said constant current generating
means to change said operating point.
3. Apparatus according to claim 1, wherein said means for adjusting
further includes:
a switch for selecting among a first operating point which
corresponds to said first Curie temperature and a second operating
point which corresponds to said second Curie temperature.
4. Apparatus according to claim 3, wherein said detecting means
further includes:
means for detecting an operating point as a function of material
resistance.
5. Apparatus according to claim 3, wherein said detecting means
further includes:
means for detecting an operating point as a function of power
supply from the constant current generating means.
6. Apparatus according to claim 1, further including:
said core being formed of copper, said first layer of said at least
two layers being formed of a first alloy having a first Curie
temperature and said second layer of said at least two layers being
formed of a second alloy different from said first alloy and having
a second Curie temperature.
7. Apparatus according to claim 6, further including:
said first layer of said at least two layers being formed of Alloy
34, and
said second layer of said at least two layers being formed of Alloy
31.
8. Apparatus according to claim 6, further including:
said core being cylindrically shaped, with said at least two layers
being formed concentrically around said core.
9. Apparatus according to claim 1, wherein said constant current
generating means further includes:
means for varying the frequency of said constant current to select
a switching ratio between selective production of heat at one of
said first Curie temperature and said second Curie temperature.
10. Apparatus according to claim 1, wherein said constant current
generating means further includes:
means for varying a pulse width of said constant current to select
a switching ratio between selective production of heat at one of
said first Curie temperature and said second Curie temperature.
11. Apparatus according to claim 1, wherein said core further
includes:
first and second sides, both of said at least two layers of
magnetic material being arranged on said first side of said
magnetic core.
12. Apparatus according to claim 11, further including:
at least two additional layers of magnetic material arranged on a
second side of said magnetic core, said at least two additional
layers having characteristics which balance characteristics of said
at least two layers arranged on said first side.
13. A heater formed as a structure comprising:
at least one electrically conductive, non-magnetic material having
at least a first side; and
at least two layers of magnetic material arranged on said first
side, a first of said at least two layers having a first Curie
temperature and a second of said at least two layers having a
second Curie temperature different from said first Curie
temperature, said at least one electrically conductive,
non-magnetic material being cooperatively arranged with said first
layer and said second layer to establish first and second operating
points, as a function of reflected resistance, which selectively
produce heat at said first Curie temperature and said second Curie
temperature.
14. Apparatus according to claim 13, further including:
said at least one electrically conductive, non-magnetic material
being formed of copper, said first layer of said at least two
layers being formed of a first alloy having a first Curie
temperature and said second layer of said at least two layers being
formed of a second alloy different from said first alloy and having
a second Curie temperature.
15. Apparatus according to claim 14, further including
said first layer of said at least two layers being formed of Alloy
34 and said second layer of said at least two layers being formed
of Alloy 31.
16. Apparatus according to claim 15, further including:
said core being cylindrically shaped, with said at least two layers
being formed concentrically around said core.
17. An apparatus for generating a heat supply comprising:
means for selectively producing heat at any one of plural
relatively constant temperature operating points of a multilayer
structure, each of said operating points being produced by a
separate material having a Curie temperature which corresponds to
one of said plural constant temperature operating points of said
multilayer structure; and
means for controlling said heat producing means at one of said
operating points as a function of reflected resistance of the heat
producing means.
18. Apparatus according to claim 17, wherein said controlling means
further includes:
means for generating a constant current, said electrical properties
being a function of material resistance.
19. Apparatus according to claim 18, wherein said constant current
generating means further includes:
means for detecting an operating point of said heat producing
means;
means for controlling said constant current generating means to
maintain operation at said operating point; and
means for adjusting said constant current generating means to
change said operating point.
20. Apparatus according to claim 19, wherein said means for
adjusting further includes:
a switch for selecting among a first operating point which
corresponds to said first Curie temperature and a second operating
point which corresponds to said second Curie temperature.
21. Apparatus according to claim 20, wherein said constant current
generating means further includes:
means for varying the frequency of said constant current to select
a switching ratio between selective production of heat at one of
said first Curie temperature and said second Curie temperature.
22. Apparatus according to claim 20, wherein said constant current
generating means further includes:
means for varying a pulse width of said constant current to select
a switching ratio between selective production of heat at one of
said first Curie temperature and said second Curie temperature.
23. Method for generating a heat supply comprising the steps
of:
selectively producing heat at any one of plural relatively constant
temperature operating points of a multilayer structure, each of
said operating points being produced by a separate material having
a Curie temperature which corresponds to one of said plural
constant temperature operating points of said multilayer structure;
and
controlling said separate Curie temperature materials at one of
said operating points as a function of reflected resistance during
selective heat production.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an apparatus and method
for generating heat, and more particularly to a method and
apparatus for providing plural controlled temperatures using
multiple layers of Curie temperature materials.
2. State of the Art
Devices for providing a regulated supply of heat are known. One
such device is described in commonly assigned U.S. Pat. No.
4,752,673 (Krumme) which discloses an auto-regulating, electrically
shielded heater. The disclosed heater of the '673 patent provides
auto-regulated heat at a single regulated temperature. Exemplary
embodiments employ a non-magnetic conductive material sandwiched
between two magnetically permeable materials of different Curie
temperatures to provide a heating surface which can be operated at
the single, regulated temperature.
FIG. 3 of the '673 patent illustrates a soldering iron which
exploits a "skin effect" to provide the single, regulated
temperature. As described in the '673 patent, the FIG. 3 soldering
iron includes an electrically conductive, non-magnetic intermediate
layer 6. The intermediate layer 6 is sandwiched between an inner
magnetic layer 2 used to provide a single regulated temperature
heating surface and an outer magnetic layer 4 used to provide
electromagnetic shielding. The inner layer 2 is illustrated as an
inner cone formed of high permeability, high resistivity, low Curie
temperature material such as an NiBalFe alloy. The outer layer 4 is
illustrated as an outer cone formed coaxial with and about the
non-magnetic intermediate layer 6 and the inner cone 2. The outer
cone 4 can be fabricated from a high permeability, low resistivity,
high Curie temperature material such as low carbon steel, cobalt or
nickel. A constant current AC supply 12 is connected between a
center conductor 8 formed of copper and large diameter ends of the
inner and outer cones 2 and 4.
In operation, alternating current from supply 12 is confined to a
surface of the inner cone 2 adjacent to the return path via the
conductor 8. Power dissipation is determined by the equation:
P=I.sup.2 R.sub.1 where I.sup.2 is a constant K due to use of a
constant current supply, and R.sub.1 is a resistance of the inner
cone 2 at the frequency of the current supply. Resistance of the
inner cone 2 is a function of the material resistivity and the
cross-section of the inner cone 2 to which the current is confined
by the skin effect. Resistance is an inverse function of
cross-sectional area so that as the cross-section of the cone to
which the current is confined decreases due to an increase in skin
effect, the resistance of the inner cone 2 increases.
The formula for skin depth in a monolithic material is: skin
depth=(5030) times the square root of (.rho./.mu.f), or
5030.sqroot.(.rho./.mu.f) centimeters where .rho. is resistivity,
.mu. is magnetic permeability and f is the frequency of the
constant current supply. Thus, skin depth decreases with increased
frequency, while effective resistance increases.
As described at column 7, line 38 of the '673 patent, when current
is initially applied to the FIG. 3 soldering iron, current is
confined to the inner cone 2. The inner cone 2 is of an exemplary
thickness which corresponds to one skin depth of Alloy 42 at 90
hertz (Hz). The device heats until the Curie temperature of the
inner cone 2 material is attained (e.g., approximately 325.degree.
C.). Once this temperature is achieved, the permeability of the
inner cone 2 material decreases and current begins to spread into
the intermediate layer 6 and the outer cone 4. The temperature of
the material of the outer cone 4 is well below its Curie
temperature and the current is therefore confined to the inner cone
2, the intermediate layer 6 and to a few skin depths of the outer
cone 4 at 90 Hz.
In other words, as the Curie temperature of the inner cone 2 is
attained, its magnetic permeability rapidly decreases and current
spreads into the intermediate layer 6 and into the outer cone 4.
Thus, the total resistance of the structure due to the presence of
the highly conductive intermediate layer 6 drops dramatically to
provide a high auto-regulating ratio. Further, most of the current
is confined to the highly conductive intermediate layer 6 and only
a small percentage penetrates the outer cone 4. The outer layer 4
is therefore only 3-5 skin depths thick to effect virtually
complete shielding of the device. This permits a large
auto-regulating power ratio to be realized in a relatively small
device using a low frequency source (e.g., 50 Hz to 10 kHz).
U.S. Pat. No. 4,701,587 (Carter et al), U.S. Pat. No. 4,695,713
(Krumme) and U.S. Pat. No. 4,256,945 (Carter et al) also relate
generally to structures which exploit an auto-regulating feature to
provide single temperature heating surfaces. Despite the
significant advantages realized by the methods and apparatus
described in these patents, they are primarily directed to
generating accurate control at a regulated fixed temperature. It
would therefore be desirable to exploit advantages of these patents
to achieve control at any one of plural user selected
temperatures.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to using an
auto-regulating feature to provide a heating structure which can be
controlled to selectively produce heat at any one of plural
regulated temperatures, without sacrificing precision and
uniformity with which any of the selected temperatures is
maintained. In accordance with exemplary embodiments, multiple
layers of alloy having different Curie temperatures, are separately
accessed as an outer most layer is heated through its Curie point
to select one of the plural auto-regulated temperatures. To select
a desired layer and temperature of operation, power to the device
can be controlled by varying the frequency of the circulating
current. By selecting an appropriate layer of Curie point material,
a heating system can provide a heating surface which is accurately
controlled to any one of plural, relatively constant regulated
temperatures.
Exemplary embodiments of the invention include means for generating
a constant current; and means for producing heat at any one of
plural relatively constant temperatures in response to said
constant current generating means. Exemplary embodiments of the
heat producing means include at least one electrically conductive,
non-magnetic material; and at least two layers of magnetically
permeable material, a first of said at least two layers having a
first Curie temperature and a second of said at least two layers
having a second Curie temperature different from said first Curie
temperature, said non-magnetic material being cooperatively
arranged with said first layer and said second layer to selectively
produce heat at a temperature selected from among said first Curie
temperature and said second Curie temperature.
Exemplary embodiments further relate to a heater comprising a core
having at least one electrically conductive, non-magnetic material;
and at least two layers of magnetically permeable material, a first
of said at least two layers having a first Curie temperature and a
second of said at least two layers having a second Curie
temperature different from said first Curie temperature, said core
being cooperatively arranged with said first layer and said second
layer to produce heat at a temperature selected from among said
first Curie temperature and said second Curie temperature.
Additional embodiments relate to an apparatus for generating a heat
supply comprising means for selectively producing heat at any one
of plural, relatively constant temperature operating points, each
of said operating points being produced by a separate material
having a Curie temperature which corresponds to one of said plural,
relatively constant temperature operating points; and means for
controlling said heat producing means at one of said operating
points in response to electrical properties of the heat producing
means.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention can be further understood with reference to
the following description and the appended drawings, wherein like
elements are provided with the same reference numerals. In the
drawings:
FIG. 1 shows an exemplary embodiment of a heater structure in
accordance with the present invention;
FIG. 2 shows a graphical representation of reflected resistance
versus temperature for a dual temperature heater structure;
FIG. 3 shows a heater structure by which reflected resistance
versus temperature curves can be obtained while the heater
structure is cooled;
FIG. 4 shows a graphical representation of reflected resistance
versus temperature for a dual temperature heater in accordance with
an exemplary embodiment of the present invention;
FIG. 5 shows an alternate embodiment of a heater structure having a
symmetrical configuration in accordance with the present
invention;
FIGS. 6a and 6b show graphical representations of reflected
resistance versus temperature for dual temperature heater
structures in accordance with the present invention; and
FIG. 7 shows an exemplary embodiment of a dual temperature current
control.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows an exemplary embodiment of an apparatus for generating
heat, the apparatus being formed as a heater structure which
includes a single construction, laminated structure 100 which can
be operated at plural, relatively constant, regulated temperatures
using layers of different Curie temperature materials. The
exemplary structure of FIG. 1 can be formed by depositing or
laminating any number of multiple layers of alloy, each of which
can have a different Curie temperature, together into a single
plate construction. One of the layers of material having a given
Curie temperature can be accessed as an outermost layer of the
heater structure is heated through its Curie point. In addition to
lamination or deposition of the multiple layers of the FIG. 1
embodiment, those skilled in the art will recognize that any number
of different techniques can be used to form the structure
illustrated. For example, hot or cold rolling, extrusion, cladding,
metallurgical techniques and so forth can also be used.
Power can be applied to the plate structure directly or through an
inductive coupling. The selection and regulation of temperature at
any one of plural predetermined operating points can be controlled
as a function of electrical properties of the plate structure
(e.g., resistance, power dissipation, or any property which is a
function of electrical resistance). The selective operation at one
of the available temperatures can be achieved by selecting
thicknesses of materials used (i.e., to establish a fixed operating
point of the FIG. 1 plate structure for a given power supply).
Alternately, selective control can be achieved by operating the
power supply (e.g., adjust frequency or pulse width) to change the
operating point of the FIG. 1 structure. By selectively varying the
power, the electrical properties of the plate structure will alter
the operating point to redistribute current within the multi-layer
plate structure and change the material layer currently operating
at its Curie temperature.
The power supply used in the exemplary embodiment of FIG. 1 can be
a "smart" power supply which is controlled in response to detected
properties of the multilayer structure to latch a predetermined
operating point. When one of the plural predetermined operating
points has been selected by adjusting the power supply, a
relatively constant temperature can be maintained by controlling
operation at the selected operating point using known techniques
which need not be described here in detail (e.g., in a manner as
described in the aforementioned U.S. Pat. No. 4,752,673, the
disclosure of which is hereby incorporated by reference in its
entirety).
Further details of exemplary embodiments will now be provided.
Referring to FIG. 1, a heater structure 100 is illustrated which
includes a core formed of at least one electrically conductive,
non-magnetic material 102 having at least a first side. In
accordance with an exemplary embodiment, the core layer 102 can be
any highly conductive, non-magnetic material (e.g., aluminum,
copper and so forth). Further, the heater structure 100 includes at
least two layers of magnetic material, such as layers 104 and 106,
formed on said first side. In the exemplary FIG. 1 embodiment, a
first layer 104 has a first magnetic permeability .mu..sub.1, a
first reflected resistance R.sub.1, a first Curie temperature
T.sub.1 and a first resistivity .zeta..sub.1, while the second
layer 106 has a second magnetic permeability .mu..sub.2, a second
reflected resistance R.sub.2, a second Curie temperature T.sub.2
and a second resistivity .zeta..sub.2. Reflected resistance is a
function of power supply frequency and material temperature.
The core 102 is cooperatively arranged with the first and second
layers to produce heat at a temperature selected from among the
first Curie temperature and the second Curie temperature. As
referenced herein, the phrase "cooperatively arranged" refers to
placement of the core relative to the magnetic layers such that
electrical current can pass directly or inductively into the
magnetic layers until the selected operating point is reached. The
plate structure can then be selectively controlled to operate at
either one of the Curie temperatures T.sub.1 or T.sub.2.
In accordance with alternate embodiments, the FIG. 1 heater
structure can further include additional layers 108 and 110. These
additional layers can be magnetic layers, having magnetic
permeabilities of .mu..sub.3 and .mu..sub.4, respectively and
having associated Curie temperatures T.sub.3 and T.sub.4,
respectively (e.g., with T.sub.4 >T.sub.3 >T.sub.2). Thus, an
inclusion of layers 108 and 110 in the exemplary FIG. 1 embodiment
represents an ability of the present invention to include any
number of magnetic layers. Each of these additional layers can have
its own independent Curie temperature which can be selected to
operate the heater structure at additional Curie temperatures
associated with the materials used.
The FIG. 1 heater structure can be controlled to selectively
operate at any one of the Curie temperatures associated with the
various materials used to form the plate, and can be used in any
number of products. For example, such a structure can be used to
provide multiple temperature soldering tips, with a low temperature
being selected for use with low temperature solder and with a high
temperature being selected for high temperature solder.
Alternately, a heater structure as illustrated in FIG. 1 can be
used in cooking grills to provide a heating surface selectively
operable at any one of plural, relatively constant temperatures for
cooking various types of food. In this manner, the plate structure
can be used as a cooking griddle plate similar to that described in
commonly-assigned U.S. Pat. application Ser. No. 07/745,843
entitled "Rapid Heating, Uniform Highly Efficient Griddle," filed
Aug. 16, 1991, but can provide operation at plural
temperatures.
In accordance with exemplary embodiments, a controllable switch is
provided to select a temperature setting which corresponds to the
effective Curie temperature of a layer included in a plate
structure. Such a switch can be a user or factory controlled switch
that controls power to coils 112 for inducing current in the plate
structure. The coils 112 can be included in insulation 111. In the
case of a heater structure having two layers of different Curie
temperatures, the switch can be set to a T.sub.2 setting to select
a higher temperature. Alternately, the switch can be set to a lower
temperature T.sub.1 setting.
The available switch ratio (i.e., the resistance versus temperature
operating characteristics of the heating structure) is limited to
the ratio of the skin depth below and slightly above T.sub.1 for
heater structures, where T.sub.2 is greater than T.sub.1. When the
actual temperature T is less than T.sub.1, skin depth is equal to
5030.times..sqroot.(.rho./.mu.f) cm, and when T is greater than
T.sub.1, .mu..sub.1 can be considered equal to 1 such that skin
depth is equal to 5030.times..sqroot.(.rho./f) cm (i.e., where
above the Curie temperature the magnetic permeability is
approximately 1 and the ratio is approximately 20 ohms:1 ohm before
switching current enters the second layer).
When the switch is set for operation at the lower temperature
T.sub.1, a current is constrained in the first low temperature
layer 104. To ensure operation at the operating point associated
with this temperature, the "smart" power supply of FIG. 1 includes
means for detecting an operating point of the plate structure as a
function of electrical properties of layers included in the plate
structure. For example, the detecting means can include means for
monitoring reflected resistance, or any derivative thereof, to
reduce the power output of the power supply until the reflected
resistance reaches a stable equilibrium.
A stable auto-regulated equilibrium can be achieved by controlling
the applied voltage from the power supply to maintain operation at
T.sub.1 . After detecting relative stability in reflected
resistance despite a continuing increase in the power supply (e.g.,
by increasing frequency or duty cycle of the power supply voltage),
detection of a relatively small decrease in reflected resistance
will cause the power supply to limit the power beyond what would
otherwise be produced until the reflected resistance begins to
increase. At that point, the plate structure can be considered to
have begun to cool such that more power is output to stabilize the
plate at T.sub.1.
Thus, the system monitors reflected resistance to maintain
operation at a given operating point. The first layer 104 can be
made sufficiently thick such that when the Curie temperature of
T.sub.1 is reached, the power supply detects a change in reflected
resistance at the frequency used to select the T.sub.1 operating
point. The power supply also keeps track of which temperature
region the plates are operating in and detects whether the actual
temperature T is less than T.sub.1.
On the contrary, when the switch is set to the higher temperature
setting T.sub.2, current is permitted to spread into the second
layer 106 by increasing power even after T.sub.1 is obtained.
Control is as follows: If T is less than T.sub.1, the power
controller continues to output maximum power even when the
reflected resistance drops during passage through T.sub.1. If T is
greater than T.sub.1, but less than T.sub.2, the temperature from
the additional heating and the contribution from the second
magnetic layer 106 (T.sub.2 layer) continues to rise.
The contribution of heating from the second layer (e.g., layer 106)
can be optimized by an appropriate choice of thickness of the
second layer 106 and the frequency of operation. The thickness of
layer 104 can be selected to be less than a skin depth at the
frequency of operation used to select T.sub.1 for operation where T
is greater than T.sub.1 and less than T.sub.2. Those skilled in the
art will recognize however, that the system will work even if most
of the heat is generated in layer 104 as long as the power supply
can detect the change in reflected resistance when T passes through
the Curie temperature T.sub.1.
A change in frequency can be used to change the reflected
resistance associated with each operating point (i.e., change the
switching ratio). The system can operate under two or more
significantly different frequencies for T.sub.1 and T.sub.2, with
additional capacitance being switched into the circuit.
With regard to the control of temperature T.sub.2, when the power
supply detects that T is greater than T.sub.1 and begins to detect
a redirection in reflected resistance, the power supply again
attempts to limit power by keeping current constant. This can be
achieved, for example, by increasing frequency and/or reducing duty
cycle until ambient heat loss matches power into the system and the
reflected resistance stabilizes.
A heater structure in accordance with the FIG. 1 embodiment can be
formed by laminating a higher temperature sheet used to form the
second layer 106 (e.g., 0.015 inch alloy) to a first side of an
aluminum core layer. The aluminum core layer can, for example, be
0.090 inches thick. The 0.015 inch alloy which is laminated to the
aluminum core layer can, for example, be Alloy 35. A first layer
104 can be formed as a 0.015 inch alloy laminated to the Alloy 35
layer. The first layer can, for example, be Alloy 32. The lower
temperature Alloy 32 used to form the first layer can be chosen
with a relative thickness with respect to the Alloy 35 of the
second layer to permit detection of electrical properties (e.g.,
reflected resistance) of the Alloy 35. Alternately, a small pick-up
coil can be used to detect electrical characteristics of the Alloy
35.
FIG. 2 illustrates exemplary electrical properties (e.g.,
resistance in ohms versus temperature) for the exemplary materials
described with respect to layers 104 and 106 of FIG. 1. As
illustrated in FIG. 2, each of the magnetic layers 104 and 106
exhibits a drop in reflected resistance at a given temperature. In
accordance with the present invention, this characteristic of hi
.mu. magnetic materials is monitored and used to permit temperature
control at plural predetermined operating points (i.e., temperature
settings).
FIG. 3 illustrates a method by which reflected resistance versus
temperature curves can be obtained while the heater structure is
cooled. In FIG. 3, an aluminum layer included in a CMI annealed
plate 304 (e.g., a plate formed with sequential layers 301, 302 and
303 of Alloy 34, aluminum and Alloy 34, respectively) is used as a
core layer. The layer 302 of the Alloy 34 included in the annealed
plate 304 serves as a high temperature layer. A layer 306 can be
formed, for example, of Alloy 32 adjacent layer 302 on a first side
of the core layer as the first, relatively lower temperature
layer.
A standard pick-up coil 308 located on the first side of core layer
304 can be used to detect current induced in the plate structure by
a power source 314 (e.g., inductively coupled coils) located on a
second side of the core. A K-type thermo-couple 310 can be used to
detect surface temperature of the structure (both the pick-up coil
and thermo-couple can be mounted within a thermal insulation
material 312).
In the exemplary FIG. 3 structure, the annealed plate 304 can
include an aluminum core 301 of 0.090 inches in conjunction with
magnetic layers 302 and 303 of Alloy 34, each having an exemplary
thickness of 0.015 inches. The layer 306 of Alloy 32 can have a
thickness of, for example, 0.015 or 0.030 inches.
Reflected resistance versus temperature curves can be obtained
while the heater structure is cooled. Resulting curves are
illustrated in FIG. 4 for cases where Alloy 32 layers of different
thicknesses are present. FIG. 4 illustrates that at low
temperatures for T less than T.sub.1 (where T.sub.1 corresponds to
the Alloy 32 Curie temperature), most of the current is restricted
to the layer 306 of Alloy 32. On the contrary, when T is greater
than T.sub.1, the current spreads into the layer 302 of Alloy 34
included in the annealed plate 304 which is below its Curie
temperature. This relatively high reflected resistance layer of the
annealed plate 304 is in parallel with the now low reflected
resistance layer 306 of Alloy 32 and an intermediate reflected
resistance can be detected.
When T reaches T.sub.2 (i.e., the Curie point of the Alloy 34 in
the annealed plate 304), the magnetic permeability of the overall
structure drops and skin depth grows until not only is the layer
302 of Alloy 34 in the annealed plate 304 conducting current, but
also the core aluminum layer is conducting current as well. The
reflected resistance now drops to a final value of approximately 1
ohm from a resistance of approximately 4 ohms per 3-4 skin depths
when T is greater than T.sub.1.
In accordance with the present invention, heat control at any
number of distinct transition temperatures T.sub.1 and T.sub.2 can
be obtained. For structures which include two operating points (for
example, the two operating points T.sub.1 and T.sub.2 determined
using the FIG. 3 structure), the reflected resistance falls rapidly
at T.sub.1 and T.sub.2 such that accurate temperature control is
possible. For example, resistance can be maintained to within plus
or minus 10% or lower of the set value. This translates into a
temperature accuracy of 107.5.+-.2.5.degree. C. at T.sub.1 and
188.5.+-.2.0.degree. C. at T.sub.2 or better.
Those skilled in the art will appreciate that the plural layers of
magnetic material in exemplary embodiments described herein can be
formed in direct contact with one another, or can be formed to
include a dielectric as an interface between layers. In alternate
embodiments, any of numerous materials can be selected with
thicknesses for achieving desired operation.
In accordance with alternate embodiments, an additional layer or
layers of magnetic material can be arranged on both sides of the
core (e.g., both the first side and a side opposite the first side
of the core) with the additional layers having characteristics
which balance the mechanical characteristics of the layers formed
on the first side. In addition, a layer can be included as a
ferromagnetic layer for shielding magnetic fields and for balancing
coefficients of thermal expansion of the various layers used to
form the heater structure.
FIG. 5 illustrates an exemplary embodiment of a heater structure
having a symmetrical design which includes a core layer 502, an
Alloy 34 layer 508 and an Alloy 31 layer 510. Symmetrically
positioned on an opposite side of the core layer 502 is a second
Alloy 34 layer 504 and a second Alloy 31 layer 506. The conductive,
non-magnetic core layer (e.g, aluminum) can be, for example, 0.090
inches thick, while the Alloy 34 layers can be each 0.015 inches
thick and the Alloy 31 layers can be each 0.018 inches thick. Using
a constant current supply with a frequency of 33 kHz, the skin
depth is small enough that most of the switching from high to low
reflected resistance occurs within the relatively low temperature
Alloy 31 with a reflected resistance ratio of 6 ohms: 2 ohms before
the current enters the higher temperature Alloy 34.
Those skilled in the art will recognize that while the foregoing
exemplary embodiments have been described with respect to
relatively planar structures, the present invention can be applied
to any structure including the soldering iron described previously.
Alternately, the present invention can be applied to cylindrical
embodiments wherein the core is formed as a wire laminated with
cylindrically shaped layers of magnetic materials. Any such number
of these materials can be included. Those skilled in the art will
appreciate that it is not the specific shape which is important to
implementing the present invention, but rather the manner by which
current passing through multiple layers of magnetic material having
multiple Curie temperatures is achieved.
FIGS. 6a and 6b illustrate dual temperature operation in accordance
with an exemplary embodiment of the present invention. At a lower
frequency of a power supply, the skin depth is larger and at the
lower temperature (e.g., 200.degree. F.), most of the switching
occurs in the low temperature layer of Alloy 31. However, as the
heater structure continues to absorb energy and heat, the lower
frequency (i.e., larger skin depth) current escapes the Alloy 34
layer into the aluminum core and provides switching at the higher
380.degree. F. Curie temperature of the Alloy 34 layer.
In accordance with exemplary embodiments, a controller can be
matched to a multi-temperature heating structure to provide precise
temperature control of multiple temperatures by adjusting
R.sub.setpoint, I.sub.constant and the power to stabilize the
heater structure. Resonant frequency can be matched with an
intermediate frequency using data obtained empirically. For
example, by setting the capacitance and inductance so that
frequency f.sub.0 is 33 kHz, then for a given temperature, a sweep
from 33 kHz under constant current up to a range of from 60 to 80
kHz can be performed. For the higher temperature of Alloy 34, a
sweep from 33 kHz down to 15 kHz can be performed. Thus, the impact
from the outer Alloy 31 layer is masked (i.e., large skin depth)
while at the lower temperature Alloy 31, sweeping from 33 kHz to 70
kHz keeps the skin depth small and out of the Alloy 34 layer.
In general, by modifying the frequency of the power supply as
described above, each of the two Curie temperature layers can be
independently selected. In an exemplary embodiment, this can be
obtained by searching and seeking final reflected resistance and by
keeping the power low enough to acquire the lower temperature Curie
point material.
Having discussed a heater structure which can provide switching
characteristics at two or more distinct temperatures, attention
will now be directed to an exemplary power supply circuit for
controlling the heater structure to select one of the plural
operating points. In accordance with exemplary embodiments, a power
supply can control power, current and reflected resistance
independently. Under normal operation, the power supply initiates a
constant current mode near or at maximum power. If reflected
resistance is relatively flat (i.e., stable) below the Curie
temperature, then current is set slightly lower than the maximum
current to provide slightly lower than the maximum power. P.sub.max
=I.sup.2 .times.R.sub.max where R.sub.max is the maximum reflected
resistance and I is constant. Once the power supply operates under
a maximum current, as a Curie temperature is reached and R begins
to decrease, power begins to decrease. In this region, the
reflected resistance is compared to a predetermined value
R.sub.setpoint. If the value R.sub.setpoint is set high, then the
current required to control at this value is near I.sub.constant.
However, if R.sub.setpoint is chosen sufficiently low, then current
will continue to be reduced until the reduced power matches the
minimum power required to maintain thermal equilibrium at this
lower resistance value.
FIG. 7 illustrates an exemplary block diagram of a dual temperature
control system for use in conjunction with the exemplary plate
structures described in accordance with the present invention. In
an exemplary embodiment, the FIG. 7 circuit can be a low frequency
resonant converter which operates in a frequency range of, for
example, 10 kHz to 100 kHz or greater.
The FIG. 7 circuit is generally designated 700 and includes a
single phase or three-phase alternating current (AC) input line
702. The input AC power is applied to input AC circuits and an
electromagnetic interference (EMI) filter 704. Outputs from the AC
circuits and filter 704 are applied to a DC bridge rectifier 706.
In the FIG. 7 example, the DC bridge rectifier can accommodate
either the single phase or three phase input. A capacitor 708 is
connected in parallel with the output of the DC bridge 706, and
voltage across the capacitor is applied to an output power stage
710. In alternate embodiments, capacitors (e.g., 1 microfarad
capacitors) can be added in parallel to each of the one-half bridge
circuits to reduce resonance frequency.
The output power stage 710 is a switching circuit for applying a
load current I.sub.load to the heater structure constituting the
load of the FIG. 7 circuit. The output load represented by the
heater structure can be a plate structure as described above with
respect to FIGS. 1-6, or can be of any desired shape (e.g.,
cylindrical, conical and so forth). In the exemplary FIG. 7
embodiment, the output load of the laminated plate structure is
represented by an output resonant circuit 712 shown to include a
capacitance 714 (labeled C.sub.e), an inductance 716 (labeled
L.sub.e) and a reflected resistance 718 (labeled
R.sub.reflected).
A current transformer 720, designated C.sub.t is coupled to the
output load of the heater structure to provide feedback to a
current control means. The current control means includes the
output power stage 710. In addition, the current control means
includes a voltage controlled oscillator 722 and an amplifier,
represented as a driver stage 724, for driving the output power
stage 710. Protection and start-up circuits 726 can be provided for
the voltage controlled oscillator and the driver stage,
respectively.
Output current from the current transformer 720 is applied to a
power detection means 730. The power detection means 730 includes a
block 732 labelled I.sub.load.sup.2 representing means for
monitoring power by calculating the square of the detected current.
The power detection means further includes a power detection block
734 designated P/I.sub.load.sup.2 for determining load. The power
detection means also includes a power input designated 738 which
supplies power to the power detection block, and can input AC power
of medium accuracy or a true value of output load power.
Equivalent resistance circuits designated 738 receive the detected
power, and are adjusted in response to operator controlled
temperature setting switches 742. Via the temperature setting
switch 742, the operator can select a first temperature setting
T.sub.1 via a contact 744, or a temperature T.sub.2 corresponding
to a resistance R.sub.2 via a temperature setting contact 746.
Outputs from the equivalent resistance circuits are applied to
voltage controlled oscillator 722 to adjust frequency output of the
voltage controlled oscillator.
In operation, a non-linear load represented by the heater structure
can be designated with a value of R.sub.reflected . The value
R.sub.reflected can generally be considered a function of frequency
and temperature as illustrated, for example, with respect to FIGS.
6a and 6b described above. To maintain a constant temperature at
any of the plural exemplary settings specified (e.g., T.sub.1 or
T.sub.2), values of reflected resistance R.sub.reflected are
determined based on the type of load used and the direction in
which the curve R.sub.reflected versus temperature is explored.
Given these parameters, the frequency of the voltage controlled
oscillator in the FIG. 7 circuit can be varied to control the
output power in a desired region. Due to the nature of the load, a
lower regulation ratio in a region near T.sub.1 (wherein T.sub.1 is
less than T.sub.2) can be obtained relative to a value
corresponding to T.sub.2.
It will be appreciated by those skilled in the art that the present
invention can be embodied in other specific forms without departing
from the spirit or essential characteristics thereof. The presently
disclosed embodiments are therefore considered in all respects to
be illustrative and not restricted. The scope of the invention is
indicated by the appended claims rather than the foregoing
description and all changes that come within the meaning and range
and equivalence thereof are intended to be embraced therein.
* * * * *